Of late, aspheric lenses or micro optical lenses used in the optical lens system have often been produced by a molding technique using a high precision die without polishing. However, the quality of a die suitable for molding is subject to various limitations from the respect of workability, durability and mass productivity. This indicates that the property of a glass to be molded is also limited. The most important property limited is a softening temperature. Molding of a glass having a softening temperature of 600 to 700° C. or higher, for example, has a large influence upon the life of a die and thus results in lowering of the mass productivity of lenses. Accordingly, it has been considered difficult from the standpoint of mass productivity to mold commercially available optical glasses of all kinds having been marketed and consequently, it becomes a subject of research to develop a glass excellent in moldability.
In JP-A-02-124743, for example, there is disclosed a low softening point, medium refractive index and low dispersion optical glass for precision molded lens, having a yielding point (At) of at most 500° C., refractive index (nd) of 1.53 to 1.62 and Abbe number (ν d) of 59.0 to 64.0, and comprising P2O5 and ZnO, as an essential element, and 28 to 49 weight % of ZnO+BaO+SrO+CaO+MgO. This optical glass has such a feature that grinding or polishing after molding is not required because of having a low yielding point (At) and excellent stability, chemical durability, as well as melting property.
JP-A-08-183632 and JP-A-11-139845 have made similar proposals, with which lowering of the softening temperature of the glass is a common subject.
Many of these glasses, however, have softening temperatures of about 400–500° C., since if lower than this temperature range, there arises a problem that chemical durability is lowered and no practical glass is obtained. When the composition of such a glass is so selected as to correspond to the optical properties of commercially available optical glasses, a sufficient softening temperature cannot be obtained sometimes.
Phosphate glasses having relatively a lower softening temperature of oxide glasses have hitherto been proposed as a low softening point glass, for example, as shown in JP-A-60-171244, JP-A-61-036136, JP-A-02-116642, JP-A-02-124743, JP-A-03-040934, JP-A-05-132339, JP-A-08-183632, JP-A-09-278479, JP-A-09-301735, etc.
The inventors have hitherto made efforts to develop a glass capable of being subjected to molding at a low temperature, i.e. at most 400° C., in particular, about 380° C. in the above described oxide glass compositions of phosphate type, and thus have found that it is possible to incorporate a considerable quantity of Al2O3 without increasing the quantity of P2O5 while increasing in essential manner the quantity of Li2O, Na2O and K2O, whereby the above described problems can be solved. The present patent application has thus been filed (JP-A-2003-026439).
It is well known that in general, addition of fluorine is effective for lowering the softening temperature of glass. However, fluorophosphate glasses up to the present time have often been used for the purpose of realizing the optical properties thereof such as low refractive index, low dispersion property, etc., as disclosed in JP-A-60-210545 or JP-A-63-144141.
Further, a glass described in JP-A-57-027941 is known as a low softening point glass (having a low glass transformation temperature). This fluorophosphate glass is a glass having a glass transformation temperature (Tg) of about 100° C., i.e. very low sortening point glass, but meets with low productivity because of containing a large amount of tin fluorides as a low boiling point compound, resulting in more vaporization during glass melting.
Thus, this glass is not considered suitable for mass production. The glass transformation temperature of about 100° C. is not considered practical as an optical glass.
The phosphate glass described in the former JP-A-02-116642 aims at a low softening point and can contain fluorine in an amount of up to 5%, but this glass does not have an object of positively adding fluorine to lower the softening point and the fluorine is only an additive component.
JP-B-59-033545 of Corning Co. (which will hereinafter be referred to as Corning Publication) discloses a low softenting point glass of fluorophosphate type for molding, having a glass transformation temperature (Tg) of at most about 350° C. This known invention is considered to be based on finding that when Al2O3 having hitherto been considered to markedly raise the glass transformation temperature (Tg) simultaneously contains fluorides in an amount of more than 3% of F as an analytical value, Tg of the glass is not markdely raised. And this publication states that it is important for obtaining a low softening point glass excellent in durability to maintain an atomic ratio of the analytical value of F: Al within a range of 0.75 to 5 and that even in Examples, the atomic ratio of F: Al is described. In Claims of the Corning Publication, there is shown % by weight on oxide basis, but this is determined by calculation, not from the component composition, but from the analytical value of the glass after melted.
In Tables 2 and 6 of the Corning Publication, there are shown residual ratios of F when melted under each condition, according to which it is apparent that there is a large dispersion over 9.11 to 69.5% and accordingly, the evaporation quantity of F is felt very large and not even. As one of factors thereof, it is considered that there is a high possibility of a reaction with a crucible. In the invention disclosed in the Corning Publication, there are used various crucibles, for example, a crucible of SiO2 type very reactive with F or Al2O3 affecting the durability thereof, resulting in one factor that the resulting glass composition is fluctuated. Furthermore, in Examples, raw materials containing water of crystallization causing vaporization of P2O5 or F are used.
As described above, the Corning Publication does not have a sufficient disclosure, since in Claims, the analytical values of F and a part of Al and the atomic ratios of F: Al are only described and no analytical values of P, Li, Na, K, etc., which are important for lowering the softening point of the glass and have a large influence upon durability, are described, in spite of that the melting conditions are not constant and the final glass composition considered to be largely affected thereby.
Optional components, PbO and CdO are useful as a component for not only raising the refractive index but also giving a low softening point as disclosed in JP-B-40-014301 and are also used in about half of Examples of the Corning Publication, because the other components for giving a low softening point, such as F, P, Li, Na, K, etc., can be reduced by the use of PbO and CdO, which are very effective for lowering the softening point while simultaneously improving the durability. However, PbO or CdO is so poisonous that the use thereof tends to be avoided lately from optical glasses in view of the standpoint of protecting the environmental pollution, and thus it is not preferred to use PbO or CdO.
In the glass composition of phosphate type of JP-A-2003-026439 having lately been filed, the inventors have found that a considerable amount of Al2O3 can be incorporated without increasing the amount of P2O5 while increasing the amounts of Li2O, Na2O and K2O components as essential components and thus reached an invention relating to a low softening point glass excellent in durability. The present invention is achieved by aiming at further lowering the softening point and improving the durability of the former glass composition and thus incorporating F, as an essential component, thereinto.
However, the present invention is considerably similar to the invention of the Corning Publication, as a whole, since the essential components of the glass composition of the former are same as those of the latter. The difference between the present invention and Corning Invention will now be illustrated:
In the Corning Publication, there are pointed out various problems as described in Prior Art. That is, claims thereof are represented by % by weight based on oxide basis, which is based on not a range calculated from the component composition but an analytical value of the glass obtained by melting, while on the other hand, the claims of the present invention are represented by % by weight based on oxide basis, calculated from the component compositions of Examples. That is, in the present invention, claims thereof are similarly represented by % by weight based on oxide basis, and accordingly, comparison of this with that described in the Corning Publication using the representation obtained from the analytical value of the glass is difficult. Since Examples show only the analytical values of F and Al and as described in Prior Art, have a number of problems relating to evaporation such as melting conditions, such a comparison is meaningless and in other words, the Corning Publication does not have a substantial disclosure of the composition of the present invention.
Thus, the comparison will now be carried out with % by weight based on oxide basis, calculated from the component compositions of Examples. In the present invention, furthermore, PbO, CdO, etc. capable of imparting a low softening point but having poisonous property being generally pointed out are not used from the standpoint of proofing the environmental pollution, so the comparison is concerned with the compositons of Examples free from PbO, CdO, etc.
In Table 1 are shown the composition ranges of weight percents on oxide basis, calculated from Examples of the present invention and Corning Publication. In claims of the Corning Publication, the composition comprises 30 to 75 wt % of P2O5, 3 to 25 wt % of R2O, 3 to 20 wt % of Al2O3, more than 3 wt % to less than 24 wt % of F and 0.75 to 5 of F: Al. Accordingly, it is understood therefrom that this composition is largely different from the real composition range calculated from Examples.
It is apparent from Table 1 that in the present invention, the range of P2O5 is less than the lower limit of the Corning Publication, the range of R2O (R: Li, Na, K) is considerably more than the upper limit of the Corning Publication and Li2O, Na2O and K2O are all contained as essential components. The upper limit of Al2O3 is considerably more than that of the Corning Publication, although partly overlapped in composition.
This is results of finding according to the present invention that a considerable amount of Al2O3 can be incorporated without increasing the amount of P2O5 while increasing the amounts of Li2O, Na2O and K2O as essential components, and further lowering of the softening point and improvement of the durability can be realized by incorporating F as an essential component.
As described above, it is apparent that the present invention is different in composition from the invention described in the Corning Publication.
Accordingly, it is the first object of the present invention is to provide an optical glass for molding, in particular, being excellent in workability and chemical durability, and capable of being subjected to precise molding at a low temperature of, e.g., at most 400° C. and having a low transformation temperature (Tg) of at most 350° C. and a specific gravity (Sg) of at most 3.1.
It is the second object of the present invention is to provide with an excellent producibility, an optical glass having optical properties, i.e. refractive index (nd) of 1.43 to 1.55 and Abbe number (ν d) of 55 to 85.